The present invention relates to cathode ray tubes, and more specifically relates to a cathode ray tube with an electron gun wherein an electron gun diameter is enlarged and productivity is high and high precision assembly is possible.
In cathode ray tubes, particularly a high brightness cathode ray tube such as a projection type cathode ray tube, a current of an electron beam projected to a phosphor screen is made larger and acceleration voltage to be applied to the final accelerating electron is made higher and also potential of a focus electrode is made higher, thereby an image of high brightness and high definition is formed on the phosphor screen.
In order to reduce aberration of an electron beam, it has been tried to increase a diameter of a final accelerating electrode as large as possible within a range limited by an inner diameter of a neck portion.
As an electron gun wherein an electron lens diameter is enlarged, for example, that having a structure disclosed in U.S. Pat. No. 4,271,374 (corresponding to JP-B 58-31696) is used.
FIG. 23 is a side view with essential portions cutaway explaining an electron gun disclosed in U.S. Pat. No. 4,271,374 as above described, and FIG. 24 is a side view with essential portions cutaway of the electron gun in FIG. 23 rotated by 90 degrees around the tube axis direction Z--Z.
In FIGS. 23 and 24, numeral 1 designates a first grid (control electrode), numeral 2 designates a second grid, numeral 3 designates a third grid, numeral 4 designates a fourth grid (small-diameter cylinder electrode), numeral 5 designates a fifth grid (large-diameter cylinder electrode), numeral 6 designates a beading glass, and numeral 7 designates a weld portion. Also numeral 11 designates an electrode support of the first grid 1, numeral 21 designates an electrode support of the second grid 2, numeral 31 designates an electrode support of the third grid 3, numeral 41 designates a support of the small-diameter cylinder electrode 4, numeral 51 designates a support of the large-diameter cylinder electrode 5, numeral 4a designates a large-diameter cylinder portion of the fourth grid 4, numeral 4b designates a small-diameter cylinder portion of the fourth grid 4, numeral 4b-1 designates a first split cylinder portion of the small-diameter cylinder portion 4b, numeral 4b-2 designates a second split cylinder portion of the small-diameter cylinder portion 4b, numeral 5a designates a large-diameter cylinder portion of the fifth grid 5, and numeral 5b designates a small-diameter cylinder portion of the fifth grid 5. In addition, although not shown, a cathode structure is housed within the fifth grid 1.
The electron gun of this type is provided with a main lens comprising the large-diameter cylinder electrode 5 composed of the large-diameter cylinder portion 5a and the small-diameter cylinder portion 5b, and the small-diameter cylinder electrode 4 composed of the large-diameter cylinder portion 4a and the small-diameter cylinder portion 4b. The large-diameter cylinder portion 4a of the small-diameter cylinder electrode 4 is inserted in the large-diameter cylinder electrode 5, and the electrode support 51 fixed on the outer surface of the small-diameter cylinder 5b of the large-diameter cylinder electrode 5 and the electrode support 41 fixed on the outer surface of the small-diameter cylinder electrode 4 are embedded in the molten beading glass 6 and fixed so that both electrodes are in desired spaced relationship. In addition, also the first, second and third grids 1, 2, 3 are fixed by embedding the electrode supports 11, 21, 31 of the grids 1, 2, 3 into the molten beading glass 6.
In the electron gun of this type since a diameter of the large-diameter cylinder portion 4a of the small-diameter cylinder electrode 4 is made larger than that of the small-diameter cylinder portion 5b of the large-diameter cylinder electrode 5, the effective lens diameter is increased and high resolution is obtained.
In the electron gun as described above, a diameter of the large-diameter cylinder portion 4a of the small-diameter cylinder electrode 4 is larger than that of the small-diameter cylinder portion 5b of the large-diameter cylinder electrode 5, and a distance between ends of a pair of electrode supports 41 fixed diametrically opposed on the outer surface of the small-diameter cylinder electrode 4 is larger than a diameter of the small-diameter cylinder portion 5b of the large-diameter cylinder electrode 5.
FIG. 25 is a sectional view taken on XXV--XXV of FIG. 23 explaining the mutual positional relationship of the large-diameter cylinder electrode 5 and the small-diameter cylinder electrode 4, and FIG. 26 is an explanation diagram of an assembling work of the large-diameter cylinder electrode 5 and the small-diameter cylinder electrode 4.
That is, since a distance between ends of the supports 41 of the small-diameter cylinder electrode 4 is larger than a diameter of the small-diameter cylinder portion 5b of the large-diameter cylinder electrode 5, in the assembly of the electron gun, electrodes other than the first split cylinder portion 4b-1 constituting the small-diameter cylinder electrode 4 are fixed by embedding the electrode supports 11, 21, 31, 41 and 51 fixed on individual electrodes into the beading glass 6 (refer to FIGS. 23 and 24), and then the first split cylinder portion 4b-1 of the small-diameter cylinder electrode 4 is inserted in the small-diameter cylinder portion 5b from the front side of the large-diameter cylinder electrode 5 and the end 7a of the first split cylinder portion 4b-1 is fixed by welding at a welding portion 7 to the end 7b of the second split cylinder portion 4b-2 fixed already to the beading glass 6.
Therefore in comparison with an electron gun of a type wherein all electrodes can be fixed at the same time, a problem exists in that positioning work and welding work are added to the work during electron gun assembly and the assembling work is complicated and maintaining of the positional precision between individual electrodes is difficult.
In order to solve the problem, a method may be thought that before fixing the electrode supports 41 to the small-diameter cylinder electrode 4, the small-diameter cylinder portion 4b of the small-diameter cylinder electrode 4 is passed through the small-diameter cylinder portion 5b of the large-diameter cylinder portion 5b of the large-diameter cylinder electrode 5 and then the supports 41 of the small-diameter cylinder electrode 4 are attached to the small-diameter cylinder portion 4b of the small-diameter cylinder electrode 4. In this method, however, a process of assembling the large-diameter cylinder electrode 5 and the small-diameter cylinder electrode 4 beforehand is separately required, and also there is difficulty in transporting the half-assembled electrode parts.
In the electron gun of this type wherein a diameter of the large-diameter cylinder portion 4a of the small-diameter cylinder electrode 4 is made larger than that of the small-diameter cylinder portion 5b of the large-diameter cylinder electrode 5, the effective lens diameter is large and as a result, a high resolution cathode ray tube can be obtained.
In the electron gun, however, in order to decrease the lens magnification, a length of the small-diameter cylinder electrode 4 serving as the fourth grid must be lengthened in the tube axis direction.
As explained in FIG. 26, a small-diameter cylinder electrode 4 in the prior art is manufactured in such a way that a first split cylinder portion 4b-1 and a second split cylinder portion 4b-2 are manufactured separately and these are made integral component by welding. In this constitution, precision of parts must be secured in part A being the large-diameter cylinder portion 4a of the first split cylinder portion 4b-1, part B being a part 4b-1 of the small-diameter cylinder portion 4b and part C being the second split cylinder portion 4b-2, i.e., the remaining part of the small-diameter cylinder portion 4b respectively. Therefore a problem exists in that manufacturing work of parts in high precision is required and work in high precision is required also during incorporating the electron gun.
In order to solve this problem, it has been tried to manufacture the whole small-diameter cylinder electrode as an integral part.
FIG. 27 is a side view partly in section showing another configuration example of a small-diameter cylinder electrode 4 wherein the large diameter cylinder portion 4a and the small-diameter cylinder portion 4b of the small-diameter cylinder electrode 4 are drawn integrally from an electrode material.
In this configuration, however, although the electron gun assembling work is improved, since the part B being the small-diameter cylinder portion 4b becomes long, it is difficult to secure the uniform precision along the length of the part B and a problem exists in that the part B is liable to be deformed during transportation.